What is a Union Word™?

_{This is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.}

After a long period of division, tends to unite; After a long period of union, tends to divide. This has been so since antiquity. – The Three Kingdoms

If a word conforms to a special rule, I call it an Union Word™.
Use the following examples below to find the rule.

And, if you want to analyze, here is a CSV version:

Union Words™,Not Union Words™
ARGUMENT,PARAMETER
DEUCE,DRAW
PANASONIC,HITACHI
CANESUGAR,HONEY
FRAGMENT,SCRAP
NORTH,SOUTH
BRANCH,DIVISION
INDEX,EXPONENT
BELLY,BACK
SAUCEPAN,WOK

Hint 1: